The camera module of shallow form factor devices such as smartphones, security cameras, automotive system cameras, industrial inspection systems, or some video or photographic devices is an important element of the device. However, the camera module remains an optical design challenge because of the requirement for a shallow form factor, the optical materials available and cost constraints.
Typically lens f-numbers for miniature camera modules of the type used in smartphones range from f/2.0 to as low as f/1.5, for example, as disclosed in: The-Verge, 27 Feb. 2018, “The galaxy S9's dual-aperture camera is great marketing”, with the trend being to attempt to increase the aperture size to enable an increase in angular resolution and so provide higher resolution images. Nonetheless, while larger aperture stops improve the Signal to Noise Ratio (SNR) in acquired images, this can be at the expense of increased optical aberration. In other applications, such as action cameras, vlog cameras, wherein the required resolution may not be as high as for still images, making wider-aperture and so faster lenses is also desirable.
Typically, optical systems for miniature camera modules feature a frontal (i.e. furthest from the image sensor) or second element aperture stop and typically 5 to 6 lens elements, the last one closest to the image sensor functioning as a field flattener, for example, as disclosed in: US 2017/0276909 and T. Steinich and V. Blahnik, “Optical design of camera optics for mobile phones,” Adv. Opt. Technol. 1, 51-58 (2012). The frontal position of the aperture stop reduces the total length of the system at the expense of aberration correction.
The lens elements for such optical systems are typically molded polymer materials, rather than glass, which is typically ground to shape. It can be both easy and cheap to produce highly aspherical surfaces suitable for aberration correction, nonetheless, the polymer materials used in such lens elements offer less diversity in refractive index and Abbe number when compared with glasses.
Curved image sensors remove or reduce the need for field flattening of the image plane. C. Gaschet et al, “Curved sensors for compact high-resolution wide field designs,” in “Novel Optical Systems Design and Optimization XX,” vol. 10376 (International Society for Optics and Photonics, 2017), vol. 10376, p. 1037603; and B. Guenter et al., “Highly curved image sensors: a practical approach for improved optical performance,” Opt. express 25, 13010-13023 (2017) disclose wide-aperture non-symmetrical optical systems adapted for such sensors. D. Reshidko and J. Sasian, “Optical analysis of miniature lenses with curved imaging surfaces,” Appl. Opt. 54, E216-E223 (2015) disclose that such systems can reach an f/1.2 aperture while remaining compact.
Referring now to FIG. 1, a “double-Gauss” optical system 100, such as are commonly employed in digital single lens reflex (DSLR) systems, is shown. Such systems comprise a central aperture stop 102 and, due to this symmetry, can balance aberrations and can compensate most of the coma, distortion and lateral chromatic aberration within the system. Indeed, in a perfectly symmetrical system, odd Zernike polynomials are compensated almost perfectly. H. Gross et al, “Handbook of optical systems”, vol. 4 (Wiley-Vch Weinheim, 2005) discloses such a double-Gauss system used for f/1.0 or larger apertures. Double-Gauss lenses usually come with a minimum of 6-elements, typically with two cemented doublets 104a, 104b around the aperture stop. Extra lens elements 106 at the back of the optical system correct aberrations. Nonetheless, such systems have a very large back focal length in order to accommodate for the mirror used in DSLR systems.
It will be appreciated that in applications requiring a shallow form factor such as smartphones, such large back focal distances are not acceptable.